gitweb/Makefile: define all .PHONY prerequisites inline

Move the '.PHONY' definition so that it's split up and accompanies the
relevant as they're defined. This will make a subsequent diff smaller
as we'll remove some of these, and won't need to re-edit the
now-removed '.PHONY' line.

Signed-off-by: Ævar Arnfjörð Bjarmason <redacted>
Signed-off-by: Junio C Hamano <redacted>